The present invention relates to a process for manufacturing plasma display panels with distributed getter material; the invention relates also to the displays obtained according to the process of the invention.
Plasma display panels are known under the abbreviation PDP, which will be used in the following.
A PDP is composed of two planar glass parts, a front one and a rear one, sealed at their perimeter by a low-melting point glass paste. In this way, between the two glass parts a closed space is formed, filled with a rare gas mixture and comprising functional components, as specified in the following; generally the rare gas mixture is composed of neon and xenon, with the latter being present in a quantity between about 4 and 15%.
The working principle of a PDP is based on the conversion into visible light, by the so-called phosphors, of ultraviolet radiations when an electric discharge is generated in the rare gas mixture. In order to form an image, a plurality of light sources of small dimensions is necessary, and thus a plurality of electrodes which generate localized discharges. Every light source formed in this way is defined in the field “pixel.”
FIGS. 1 and 2 show in cross-section, respectively, a part of a known PDP and of its front glass panel only (the relative dimensions are not in scale); in particular, the two views are taken along two mutually orthogonal sections. On the front glass panel, FP, is present a series of pairs of parallel electrodes, E1 and E2, defined as supporting electrodes and as scanning electrodes respectively, being protected by a dielectric layer, DF, which in turn is covered with a layer, M, of magnesium oxide (MgO). This layer, M, has the double function of protecting the dielectric layer from the ionic bombardment due to the plasma triggered by the discharge, and of supplying secondary electrons for maintaining the discharge. On the rear glass panel, RP, a series of so-called address electrodes, AE, is present (having a direction orthogonal to the electrodes E1 and E2), covered by a dielectric layer, DR. A series of barriers R (known in the field as “ribs”) that are mutually parallel to each other and parallel to the electrodes AE, is constructed onto this latter layer. Since the internal pressure of the display is lower than atmospheric pressure, the upper portion of the ribs is in contact with layer M, thus dividing the inner space of the display into parallel channels, indicated as C in the drawing, having a width between 0.1 and 0.3 mm. Each one of these channels is covered internally with phosphors. Particularly, in the channels there are present in an alternating way phosphors, able to convert ultraviolet light respectively into red (phosphors PR), green (PG) and blue (PB) visible light. By applying a potential difference to a given electrode pair E1 and E2 and to an electrode AE, an electric discharge is generated in the zone of a pixel, that causes the light emission indicated by the arrows in FIG. 1. The area of the front glass panel, corresponding to the zone of the channels, is the part on which the image is formed.
Recently, interfering effects between the electric discharges at contiguous pixels in one channel have been noticed (a phenomenon known as “cross-talking”), which cause a deterioration of the image quality, especially in the case of high-definition displays (i.e. having pixels of small dimensions). In order to reduce the phenomenon, more complex configurations of the ribs have been proposed, such as shown in the FIGS. 3 to 5. In the case of FIG. 3 the channels are divided transversally by barriers of a height that is lower than that of the ribs; in the case of FIG. 4 the ribs define pixels of essentially hexagonal geometry, separated by necks with a reduced cross section; finally, FIG. 5 shows a structure in which there are transversal barriers of the same height as the ribs, so that the inner space of the display results divided in ordered rows of completely closed cells (each one corresponding to a pixel).
The manufacturing processes of PDPs are essentially of two types, i.e. the so-called “pumping tubulation” processes, currently used, or the “chamber processes”, under investigation. In a process of the pumping tabulation type, in one of the two glass panels forming the display (usually the rear panel) an opening is formed, connected to a glass tubulation. After the perimetral sealing of the perimeter of the two glass panels, first the evacuation of the inner space is carried out by pumping through the tubulation, and subsequently the inner space is filled with the desired rare gas mixture; finally, the tubulation is closed by compression under heat, thus sealing the inner space of the display. In contrast, in a chamber process the two finished glass panels are introduced into a chamber filled with an atmosphere having a composition and pressure corresponding to that of the rare gas mixture required for operating the PDP, and are sealed to each other in this chamber, to enclose the appropriate atmosphere. Consequently, in the case of the pumping tabulation processes the filling of the display with the gas mixture follows the sealing of the two glass panels, while in the case of the chamber processes the two steps are simultaneous. It must be noted that while generally the choice of either process is free, in the particular case of displays with an internal structure with closed cells, as that shown in FIG. 5, it is necessary to resort to the chamber process, because after the sealing of the two glass panels it would no longer be possible to evacuate the cells or to fill them with the rare gas mixture via the tubulation.
For proper operation of these devices it is necessary that the chemical composition of the gaseous mixture in which the plasma is formed remains constant: in fact, the presence in the gaseous mixture of traces of atmospheric gases, such as nitrogen, oxygen, water or carbon oxides, has the effect of varying the operating electrical parameters of the PDP, as discussed in the articles “Effect of reactive gas dopants on the MgO surface in AC plasma display panels,” by W. E. Ahearn et al., published in IBM J. Res. Develop., Vol. 22, No. 6, p. 622 (1978); “Color plasma displays: status of cell structure designs” by H. Doyeux, published in SID 00 Digest, p. 212; and “Relationships between impurity gas and luminance/discharge characteristics of AC PDP” by J.-E. Heo et al., published in “Journal of Information Display”, Vol. 2, No. 4, p. 29 (2001). In particular, among PDP manufacturers, water is the impurity regarded as the most dangerous one. These impurities can remain in the panel following the manufacturing process, or they can accumulate at the inside with time, as a consequence of outgassing of the component materials themselves. The first contribution is particularly important in the case of the pumping tabulation processes, in which the limiting factor for the evacuation speed of the inner space is the low gas conductance in the channels, which causes the problem that the removal of the impurities cannot be completed within the evacuation times (some hours) compatible with the industrial manufacturing processes of PDPs. The problem is even worse in the case of PDPs with internal structures like those shown in FIGS. 3 and 4 (while as already stated, displays with a structure of type shown in FIG. 5 cannot be produced in this way). The contribution from the outgassing during the service life is, however, the same in PDPs produced by both methods.
In order to solve these problems it has been proposed to introduce into the PDPs in various ways getter materials, i.e. materials capable of reacting with impurities and to chemically fix them, thus removing them definitely from the inner space of these displays.
U.S. Pat. No. 6,472,819, U.S. patent application publication 2003/0071579 A1, and Korean published patent application KR 2001-104469 A1 disclose PDPs in which getter material deposits are present in the peripheral zone, within the sealing zone between the front and rear glass panels and the image-forming zone. The getter deposits according to these documents are efficient both in increasing the removal speed of the impurities during the manufacturing process of the display, and in removing the impurities generated by outgassing during the service life thereof. In spite of the advantages offered, the getter systems according to these documents do not yet yield totally satisfactory results. In fact, particularly during the service life of the display, the impurities need some time to reach the getter materials, during which inhomogeneity of gas composition across the PDP may arise, and consequently differences in luminosity or in image quality at different parts of the display.
To overcome the problem, some patent documents describe various configurations in which the getter material is distributed in the image forming area.
Korean Patent No. 366095 and Korean patent application publication KR 2001-049126 A1 describe PDPs in which linear getter material deposits, parallel to the electrodes (of the type E1 and E2 in FIG. 1), are present on the front glass panel, so that the getter deposits also form the so-called “black matrix” of the display (a dark element surrounding the pixels that increases the contrast of the display). However, in the structures described in these documents the getter deposits cover part of the surface dedicated to the light emission and thus an extremely precise control of dimensions and location of these deposits is required, with quite complex manufacturing processes. Moreover, at least in the case of Korean Patent 366095, the surface of the getter deposits forms an undercut with respect to the surface of the magnesium oxide layer, whereby every getter deposit provides for a possible communication passage for the gases between contiguous channels, with a possible increase of the cross-talking.
U.S. Pat. No. 6,483,238 B1 and Japanese patent application publication JP 2002-075170 A1 disclose PDPs in which the ribs are made from a porous material containing the getter material, while the Korean patent application publication KR 2001-091313 A1 discloses a PDP in which the ribs are made from getter material. These structures, however, show some constructive problems, in so far as the ribs are generally constructed by successive depositions of a suspension of particles of the desired material with the screen-printing technique, drying after every layer deposition, and final consolidation of the rib by thermal treatment. The use of a mixture of various materials, among them a getter, gives some problems, since the getter could be contaminated during the thermal treatments of drying and consolidation by the vapors of the solvent used for the deposition, thus inactivating the getter for the service life of the display. Conversely, the presence of getter particles could compromise the mutual adhesion of the particles of ceramic material of which the ribs are normally formed, thus reducing their mechanical resistance.
Finally, U.S. Pat. No. 6,603,260 B1 discloses a PDP in which a getter material is deposited on the upper surface of the ribs, in contact with the front glass panel. However, this solution also presents notable constructive difficulties. In fact, in order to deposit the getter selectively only on the upper surface of the ribs, extremely precise masking operations are necessary to avoid the material spreading along the lateral walls and occupying the zone designated for the phosphors (or covering them, in case these are already present).